This invention generally relates to portable battery-powered equipment having a thermal recorder. In particular, the invention relates to such battery-powered equipment used to monitor patients during transport in a hospital or other patient care setting.
When providing medical care to patients, it is frequently necessary to monitor the patient using medical diagnostic instruments. One type of instrument, the patient monitor, is capable of monitoring the patient to acquire electrocardiogram data, cardiac output data, respiration data, pulse oximetry data, blood pressure data, temperature data and other parameter data. In particular, lightweight portable monitors exist which can be moved with the patient, allowing continuous monitoring during patient transport.
To facilitate monitoring at remote locations or during patient transport, modern portable patient monitors are powered by rechargeable batteries. Extended-use batteries, with quick recharge times, help maximize monitor availability. Advanced monitors have a smart battery management system which maximizes battery life, reducing maintenance and replacement. These patient monitors can also be plugged into any conventional electrical power system for use, e.g., at the patient""s bedside, before and/or after the patient is transported. At the bedside, advanced patient monitors can be hard-wired to a central station via a local area network (LAN) for enhanced patient surveillance efficiency. In addition, the most advanced patient monitors have a built-in wireless option which enables the monitor to go mobile without sacrificing connectivity. Such monitors also support importation of demographic and laboratory data from a hospital information system for increased efficiency.
Portable patient monitors with integral battery power supply are commercially available in a compact, ergonomic package which allows easy handling. Typically such monitors have a drop-tested rugged design which allows them to withstand the punishment of the demanding intra-hospital transport applications. Mounting options make these monitors ideally suited for headboard/foot-board, siderail, rollstand and IV pole use. The compact design is achieved in part through the use of flat display panels. The color or monochrome screen accommodates all numerics and multiple waveforms.
In addition to displaying waveforms and numerics representing the data being acquired, advanced patient monitors have a central processing system which stores and analyzes the acquired data. In particular, the central processing system is programmed with algorithms for analyzing the acquired data. The central processing system controls the transfer of data to the display panel for display and to the LAN via either a hardwired or wireless connection. In addition, the central processing system sends the data to a thermal recorder, which prints the data on a substrate.
Thermal recorders used in power-limited environments, such as portable battery-powered equipment, need to have a reliable means of limiting peak power demands. Typically the thermal recorder consumes a disproportionately large share of the system power. This power consumption can reach extreme levels, especially during electrocardiograph (ECG) artifacts such as lead failure (e.g., a lead falling off the patient""s chest) and when an electrosurgical unit (ESU) is being used, which produce spikes in the power consumed by the thermal recorder. The high peak power demands imposed by thermal recorders require host designers to give special consideration to the power supply. The host power supply must have a large enough capacity to deliver the required peak power, resulting in a larger, more complicated and costly power supply. These considerations present unique design problems, especially for portable equipment whose typical prerequisites are small size and low weight.
The present invention is a method and an apparatus for limiting the peak power consumed by a thermal recorder connected to portable battery-powered equipment. In accordance with the preferred embodiment, the solution to the problem of limiting the peak power involves a hardware solution contained in the battery-powered equipment combined with a software solution contained in the thermal recorder.
The hardware solution uses a filter and an electronic circuit breaker. A circuit breaker current sense resistor and an output capacitor form an RC filter and provide a large current reservoir for the thermal recorder which averages the peak current demands seen at the circuit input. The electronic circuit breaker provides a current limit function and will not allow a current greater than a predetermined amperage level to be drawn. This forces peak demands above the predetermined amperage level from the thermal recorder to be drawn from the output reservoir capacitor. If these peak demands are continuous for a set period of time, the electronic circuit breaker will trip and will remove power from the thermal recorder.
The software contained in the thermal recorder uses a pulse-width limit table. The thermal recorder operates on the principle of producing an image by burning dots onto the surface of specially coated paper that is drawn across a print head. The burning of the dots by miniature heating elements in the print head is what consumes the large amount of current. The amplitude of the current depends on the number of dots burned. The darkness of the image is controlled by the length of time the heating elements are turned on. The length of time must be varied by the thermal recorder software to maintain consistent image darkness due to external factors such as a changing supply voltage. In accordance with the preferred embodiment of the invention, the length of time the heating elements are turned on is restricted per burn cycle in order to limit peak current demands.
The invention also encompasses a method of programming a thermal recorder to limit the peak power consumed. In accordance with this method, the length of time or pulse-width limits to be applied by the thermal recorder are empirically derived from the hardware. The steps of the method are as follows. First, an electronic load is connected to the hardware. A periodic load is applied equal to the frequency of the burn cycle used by the thermal recorder. The duty cycle of the load is set to a multiplicity of different values and for each selected value, the load is slowly increased until the electronic circuit breaker is tripped and the corresponding value of the maximum current is recorded. The maximum currents along with the respective duty cycle values are then graphed and the equation which fits the graphed data is determined. This equation is then used to construct a pulse-width limit lookup table, which is stored in memory inside the thermal recorder. Incorporating multiple pulse-width limit tables can make for further enhancements to account for various host supply voltages and current limits.
When the thermal recorder calculates that required pulse width used to burn dots, it will take this value and compare it to a value pulled from the pulse-width limit table and use the lesser of the two. If the pulse width from the limit table is used, this will have the effect of lightening the dots used in this burn cycle. The dots will be lightened only to an extent required to not trip the electronic circuit breaker. Sections of the produced image that have been pulse-width limited will typically be confined to artifacts caused by ECG lead failure or ESU interference.